An ultrasonic diagnostic apparatus irradiates a biological subject, such as a human or an animal, with ultrasonic waves. The ultrasonic diagnostic apparatus detects echo signals reflected within the subject and displays a tomogram of in vivo tissue, thereby providing information required to diagnose the subject. In the ultrasonic diagnostic apparatus, an ultrasound probe is used to transmit the ultrasonic waves into the subject and to receive the echo signals from within the subject.
FIG. 9 is a cross-sectional view of a configuration example of a conventional ultrasound probe of this type. In FIG. 9, to transmit and receive ultrasonic waves to and from a subject (not shown), an ultrasound probe 30 includes a plurality of piezoelectric elements 11, an acoustic matching layer 12 (12a and 12b) that is one layer or more (two layers in FIG. 9), an acoustic lens 13, an electrical terminal 15 for signals, a backing material 14, and an electrical terminal 16 for grounding. The piezoelectric elements 11 are unidirectionally arrayed (a direction perpendicular to paper surface in FIG. 9). The acoustic matching layer 12 is provided on a surface of the piezoelectric elements 11 on a subject side (upper side in FIG. 9) (a surface on the subject side will, hereinafter, be referred to as a front surface). The acoustic lens 13 is provided on the front surface of the acoustic matching layer 12. The electrical terminal 15 for signals is provided on a surface of the piezoelectric elements 11 on a side opposite to the subject side (lower side in FIG. 9) (a surface on the side opposite to the subject side will, hereinafter, be referred to as a back surface). The backing material 14 is provided on the back surface of the electrical terminal 15 for signals. The electrical terminal 16 for grounding is mounted between a first acoustic matching layer 12a and a second acoustic matching layer 12b. 
A piezoelectric element 11 is made of a piezoelectric ceramic, such as lead zirconate titanate (PZT), a monocrystal, or a composite piezoelectric material that is a combination of the piezoelectric ceramic, the monocrystal, and a high-polymer material. Alternatively, the piezoelectric element 11 is made of a piezoelectric material made of a high-polymer material, represented by polyvinylidene fluoride (PVDF), and the like. An electrode is formed on the front surface and on the back surface of the piezoelectric elements 11. Electrical signals are transmitted and received between the electrodes and the piezoelectric elements 11. In other words, the piezoelectric elements 11 convert voltage into ultrasonic waves and transmit the ultrasonic waves into the subject. The piezoelectric element 11 also receives echoes reflected within the subject and converts the echoes into electrical signals.
The acoustic matching layer 12 is provided to efficiently transmit the ultrasonic waves to the subject and receive the ultrasonic waves from the subject. More specifically, the acoustic matching layer 12 serves to bring an acoustic impedance of the piezoelectric element 11 closer to an acoustic impedance of the subject in stages. In the example shown in FIG. 9, the first acoustic matching layer 12a and the second acoustic matching layer 12b are laminated to form the acoustic matching layer 12. Graphite, which is a conductive member, is used as the first acoustic matching layer 12a. The electrical terminal 16 is taken out from the front surface of the first acoustic matching layer 12a, the electrical terminal 16 being an insulating film on which a metal film is deposited. Furthermore, the second acoustic matching layer 12b is provided on the front surface of the electrical terminal 16. In this configuration, the insulating film does not easily break, even should the piezoelectric elements 11 break as a result of mechanical impact from an external source or the like. Therefore, electrical conductivity can be ensured and, as a result, reliability is high (refer to, for example, Patent Document 1, below).
On the other hand, a configuration is also known that achieves a broader band of frequency through use of a material having a higher acoustic impedance than graphite as the first acoustic matching layer 12a (refer to, for example, Patent Document 2, below).
Moreover, a configuration of the first acoustic matching layer 12a is also known in which a through-hole is provided on a portion of an insulating member. A conductive member is fitted into the through-hole, thereby connecting the electrical terminal provided on the front surface of the first acoustic matching layer 12a and the piezoelectric elements 11 provided on the back surface of the first acoustic matching layer 12a (refer to, for example, Patent Document 3, below).
The acoustic lens 13 serves to focus an ultrasonic beam to increase resolution of a diagnostic image. The acoustic lens 13 is an optional element, provided as required. The backing material 14 is connected so as to hold the piezoelectric elements 11, and further serves to attenuate unnecessary ultrasonic waves.    Patent Document 1: Japanese Patent Application Publication No. H07-123497    Patent Document 2: Japanese Patent Application Publication No. 2003-125494    Patent Document 3: Japanese Utility Model Application Publication No. H07-37107
In an electronic scan type ultrasonic diagnostic apparatus, the piezoelectric elements form a plurality of groups. The ultrasonic diagnostic apparatus drives each piezoelectric element group with a certain amount of delay time between there. The ultrasonic diagnostic apparatus then transmits the ultrasonic waves into the subject from each piezoelectric element group and receives echo signals from within the subject. As a result of the delay time being provided in this way, the ultrasonic beam is focused or dispersed, allowing an ultrasonic image that has a wide field of view or a high resolution to be obtained.
A system in which a plurality of piezoelectric element groups are given a constant amount of delay time and an ultrasonic image is obtained is already known as a common system. A broader band of frequency is essential in the ultrasound probe for obtaining a high-resolution ultrasonic image, such as that described above. Moreover, while high resolution is desired, the ultrasound probe is also required to have a slim form to enhance operability because the ultrasonic probe is operated by a doctor or a laboratory technician, and the diagnostic image is obtained by the ultrasound probe coming into direct or indirect contact with the subject. During operation and other instances, the ultrasound probe may become broken as a result of being accidentally dropped or struck. Therefore, high reliability against breakage is also required.
As a measure for achieving a broader band of frequency in the ultrasound probe, a configuration is given in which the acoustic matching layer provided on the front surface of the piezoelectric elements is three or more layers, as described in Patent Document 2. However, in this configuration, silicon, which is a semiconductor, is used in a first acoustic matching layer on the piezoelectric element side. Therefore, the electrical terminal taken out from the electrode of the piezoelectric elements on the first acoustic matching layer side can only be taken out from a portion of an end section of the electrode formed on the piezoelectric elements. Therefore, in this configuration, when the piezoelectric elements and the electrode break as a result of mechanical impact, disconnection occurs upon breaking, and functions deteriorate.
On the other hand, in the configuration described in Patent Document 1, graphite, which is conductive, is used as the first acoustic matching layer. The electrical terminal is provided on the front surface of the first acoustic matching layer, the electrical terminal being an insulating film on one main surface of which a metal film is deposited. Therefore, reliability is high. However, the conductive material used in the first acoustic matching layer has low acoustic impedance. Moreover, the acoustic matching layer can only be laminated to include two layers. Therefore, a broader band of frequency is difficult to achieve. Ultrasound probes of recent years tend to have broader bands. Diagnosis is often performed by a high-resolution ultrasonic image being obtained through use of second or third harmonic content of a fundamental frequency, or through use of a plurality of frequencies. Therefore, achieving a broader band of frequency is becoming increasingly important.